1. Technical Field
The present invention relates to an improved facility for the testing, designing, and manufacturing of shaped charges optimized for flow performance in natural rock, as opposed to current standardized concrete targets. Specifically, the facility is capable of providing rapid, useful feedback from stressed natural rock targets, both during the charge design process and during manufacture in response to changing conditions and variations in raw materials.
2. Description of Related Art
Perforating systems utilizing shaped explosive charges have become the dominant method for connecting a cased-and-cemented completion to the desired reservoir interval. Due to the location of oil and gas within subterranean formations, one field that has benefited greatly from these perforating systems and their explosive charges is the production of oil and gas. Perforating systems are used to fracture the subterranean formations to increase their permeability, thereby increasing the rate at which the oil or gas can flow through the formation.
Before fracturing occurs, a well is bored into the formation. Individual lengths of relatively large diameter metal tubulars are secured together to form a casing string that is positioned within a subterranean well bore to increase the integrity of the well bore and provide a path for producing fluids from the formation to the surface. Conventionally, the casing is cemented to the well bore face and subsequently perforated by detonating shaped explosive charges. These perforations extend through the casing and cement a short distance into the formation. In certain instances, it is desirable to conduct such perforating operations with the pressure in the well being overbalanced with respect to the formation pressure. Under overbalanced conditions, the well pressure exceeds the pressure at which the formation will fracture, and therefore, hydraulic fracturing occurs in the vicinity of the perforations. As an example, the perforations may penetrate several inches into the formation, and the fracture network may extend several feet into the formation. Thus, an enlarged conduit can be created for fluid flow between the formation and the well, and well productivity may be significantly increased by deliberately inducing fractures at the perforations.
A shaped charge is an explosive device within which a metal shell called a liner, often conical or hemispherical, is surrounded by a high explosive charge, enclosed in a steel case. When the explosive is detonated, the liner is ejected as a very high velocity jet that has great penetrative power. Shaped charge performance has advanced considerably as the result of more potent explosives, tighter manufacturing tolerances, improved quality control, and overall design enhancements. However, it is well known that the flow path created by perforation with shaped charges is seldom ideal.
Research and development efforts to maximize the penetration capabilities of perforating systems and charges were based largely on trial and error when they were first introduced in the early 1950's, and although they were effective, for the most part, they were quite dangerous. It was not until the 1970s that modeling codes could predict with any accuracy how a shaped charge would behave. While the concept of a metal surface being squeezed forward may seem relatively straightforward, the physics of perforating systems utilizing shaped charges is very complex and even today, researchers recognize that extensive testing is required under simulated well conditions to satisfy the industry's need to predict perforator effectiveness. Consequently, industry standardization has been applied to effectively regulate perforating systems.
The American Petroleum Institute (API) issued its first recommended practice on the evaluation of wellbore perforators in the 1980's and the revised version, issued in 2001, sets out the currently accepted guidelines for perforator evaluation. These 2001 guidelines are divided into four sections, or procedures, to be used when representing perforating equipment performance to the oil & gas industry: 1) performance under ambient conditions, using a concrete target; 2) performance into stressed rock, using a Berea sandstone target; 3) performance at elevated temperature, using a steel target (for systems destined for high temperature deployment); and 4) flow performance under simulated downhole conditions.
Due to the variability of the properties of Berea sandstone and high costs associated with testing under simulated downhole conditions, concrete targets became the more favored standard on which to base equipment selection and purchasing decisions, resulting in tighter control on concrete formulation to improve overall uniformity. Accordingly, perforator manufacturers have focused on optimizing their products for maximum penetration into the prescribed concrete target. However, predicting downhole penetration using a concrete target is widely acknowledged as problematic since penetration is influenced by many different rock properties.
Any prediction on perforation flow performance based on Section 1 data is misguided due to the different rock properties in formations as well as the many factors that contribute to the flow performance of a perforation tunnel. Tunnel performance is the result of tunnel geometry and quality. Geometric effects include tunnel length and diameter. Quality effects include the distribution of material inside the tunnel, the condition of any material inside the tunnel, the condition of the tunnel side walls, tip geometry, and tunnel cracking. In addition, factors such as shaped charge design, target composition, target stress state, the initial relative pressure state between the wellbore and the formation (under, at or over-balanced), and the dynamic pressure events during perforation will each have an effect upon the geometry produced by a given perforating system.
Perforators optimized for maximum concrete penetration are highly unlikely to be optimized for maximum rock penetration and will not result in perforation geometry conducive to maximum flow. Research has in fact shown that the flow performance of charges optimized for a stressed rock target may be 10-15% greater than similar charges optimized for a cement target, with as much as 20% greater penetration into the rock target. For example, Table 1 (below) helps invalidate the API's Section 1 procedure as a legitimate benchmark by proving that some charges that perform well when tested in rock targets perform poorly when tested in cement targets.
TABLE 1Performance of Charges Optimized for Rock or Cement.Penetration (inches)TargetCharge ACharge BCement Quality Control Target42.020.0Berea Sandstone Quality20.515.0Control TargetStressed Berea Sandstone10.611.4Target
Development of perforating systems using a concrete target is also misleading because it will drive the charge design towards the use of a suboptimal energy distribution, which is required to maximize penetration into concrete. As a result, this model of standardization forces the makers of perforating systems away from designs shown to deliver greater flow performance. To make matters worse, only 2-3 tests can be run in a normal work day under Section 2's established procedures for the testing of stressed rock targets, adding significant costs and delays when testing with rock targets versus the more accepted concrete targets currently used.
The quality control of shaped charges is also critical to ensuring product performance of perforating systems. Quality control processes provide important feedback to allow for the adjustment of the manufacturing of shaped charges in response to changing conditions and variations in raw materials. Quality control testing often leads to improved charge design, demonstrating a gradual re-optimization toward quality control target material. Predictive computer modeling can be useful in the early stages of development of a shaped charge, but an iterative process of testing and learning from design variations is the best way to achieve peak performance while gaining an understanding of the target rock formation. Consequently, there is a need for rapid testing procedures to provide for decreased operating and capital expenses in developing shaped charges. Further, there is a need to improve upon the API's Section 2 procedure for optimizing and standardizing perforating systems and charges using a stressed natural rock medium such that the flow path created by perforation of subterranean formations can be predicted more accurately. Further, there is a need for consistent production of these improved charges to achieve consistently improved well performance. Finally, there is a need for perforating systems that can also be rapidly customized to deliver maximum flow performance at specific field conditions, making region-, formation-, and even field-specific charge designs a realistic future goal.